In the art of injection molding plastic fishing lures, large presses (pneumatic or hydraulic) are typically used for molding rigid plastics, such as polystyrene and polypropylene, and smaller presses are typically used for molding flexible plastics, such as plasticized polyvinyl chloride (PVC). Flexible plastics allow the use of smaller presses due to the reduced injection pressures associated therewith. For example, typical injection pressures of at least 5000 psig are associated with injection molding of rigid plastics and typical injection pressures for flexible plastics seldom exceed 50 psig. Prior art presses normally comprise a stationary platen and a moving platen, each of which has an opposing mold section rigidly attached thereto. Clamping the mold sections together to withstand injection pressures is typically accomplished with a pneumatic or hydraulic cylinder, which forces the moving platen against the stationary platen. Precise alignment between the platens is governed by bushings that guide movement of the moving platen.
Injection molds have been designed in various shapes and sizes, with the most common design having rectangular-shaped opposing plates with a thickness between 3/4 to 1 inch, wherein the upper plate is known as a "cope" and the lower plate is known as a "drag". The two mold sections typically have multiple opposing cavities communicating with a common runner through which plastic is fed into the mold cavities. The connection between a runner and a mold cavity is known as a "gate", which is typically narrowed to facilitate removal of a lure from its associated runner. The gate should be sized to allow continuous feeding of plastic through the gate as the plastic within the mold cavity cools. Otherwise, the lure will "shrink" as it cools, a defect which manifests as an indention, usually located in the thickest portion of the lure.
In recent years, the trend in the flexible plastic lure industry has been to use larger presses and larger molds having more cavities in an attempt to reduce manufacturing costs. This trend has resulted in significant changes to operational procedures to facilitate flowing plastic longer distances through the mold. For example, in a large mold having numerous cavities, the plastic filling the first cavities will be much hotter than the plastic filling the last cavities because the plastic is cooled as it flows through the long runners. A problem that has arisen with the larger molds is that as the plastic temperature decreases across the runner/cavity system, the viscosity of the plastic increases, creating a viscosity gradient. This viscosity gradient increases the difficulty of producing quality lures, especially in a lamination process wherein two different colored plastics are required to flow side-by-side. Viscosity is the primary physical property which dictates the flowability of plastics. Thus, to assure complete filling of all mold cavities, the plastic must be heated to a higher temperature than would be required for a mold having a small number of cavities.
Injection machines have been utilized for molding plastic articles comprising a single color, two colors wherein the lure body is one color and the lure tail is another, two colors wherein the inner portion of the lure is one color and the outer portion is another (known as "cored"), and two (or more) colors side-by-side along the length of the lure via a lamination process. The term "lamination" stems from fluid mechanics wherein a laminar flow occurs whenever part of a flowing fluid remains fixed in position relative to an adjacent flowing fluid and moves in a predictable linear fashion. Viscosity control of the two plastic flows is critical to start and maintain a laminar flow.
The technology of producing single colored, multi-colored, cored, and laminated lures is dependent on the plastic flow control devices incorporated into the mold design and/or the injection nozzle. Two-colored lures are typically injected in a manner such that the body of the lure will be one color and the tail another color. Cored lures typically exhibit a transparent color on the surface with a contrasting color inside. In the past few years, the lamination process has overwhelmed the industry in popularity due to its success in the market. During a typical lamination process, two streams of plastic having different colors are flowed side-by-side into each lure cavity. Laminations with two colors can be achieved through mechanical flow separators, such as mold runner flappers or stripper bars.
Mold runner flappers serve to start the two colored plastics into a runner in a laminar flow; however, maintaining laminar flow in large molds is difficult due to the temperature/viscosity gradients previously discussed. The mold runner flappers typically render the resulting plastic runner unsuitable for recycling because of mixing of the two colors of plastic, which increases material wastes and the cost of lure manufacturing. Other shortcomings in the mold runner flapper design include poor color separation control resulting in intermixing of the plastics and, therefore, a poor quality lamination; and this design is impracticable for laminations normal to the mold parting line.
The stripper bar design, taught in U.S. Pat. No. 4,969,811, was an effort to overcome the shortcomings of other lamination techniques. The stripper bar design maintains separate runners for each of the two colored plastics and delivers them to the gate of each lure cavity. Flow into each cavity is controlled by indexing the stripper bar after the runner cavities are fill to expose the stripper bar ports to each lure cavity. Although some mixing of the two plastics may occur near the gates, the stripper bar yields quality laminated lures at reduced manufacturing costs. Nonetheless, a limitation of the stripper bar, as well as other lamination techniques, is that orientation of the lamination is limited to having the color separation plane coincident with the mold parting line.
One particularly demanding innovation has been lamination normal to the mold parting line (vertical lamination). Vertical lamination produces a lure having one color on its top (back) and another color on its bottom (belly). In most cases, a mold cavity cannot be arranged within the mold to allow vertical lamination due to undercuts which preclude normal casting or machining operations for mold making. For many years, a vertical lamination effect was accomplished by spray painting the back of the lures with a compatible paint. The additional manufacturing operations were costly and involved handling of flammable and toxic solvents. Several years ago, the stripper bar was redesigned to allow vertical laminations. Nonetheless, shortcomings of the stripper bar design are approximately 50% of the runners are unsuitable for recycling due to intermixing of the plastics near the cavity gate and maintaining laminar flow in large molds.
Other shortcomings seen in prior art designs include: (1) separate molds are required for single color, two color, cored, and lamination processes; (2) horizontal mold parting line presses do not provide a natural escape path for air inside the mold cavities as the cavities are filled with plastic, resulting in trapped air pockets along the top surface of each plastic lure; (3) each injection molding machine requires an operator to remove plastic runners and attached lures from the mold before the mold can be recycled; (4) additional labor is required to detach the individual lures from the associated runners and package the lures for shipment and/or further processing; (5) runners are either discarded, thus increasing material waste, or separated according to color and recycled, thus requiring additional labor; and (6) production cycle time is dependent on the size of the individual mold cavities and the volume of plastic injected therein because of the time required for the plastic to cool sufficiently to maintain its shape when removed from the mold cavity. Since the heat conduction rate through plastic is much lower than through the mold, which is typically comprised of aluminum, the plastic cooling period controls the cycle time. For example, when the lure is removed from the mold cavity, only the outside skin of the lure has typically been cooled. If the plastic is not allowed to sufficiently cool, the residual heat inside the lure can reheat the entire lure, making the lure receptive to deformation. Typical cycle times for injection molding machines are from 50 seconds for small lures to 200 seconds for large lures.
As can be seen from the foregoing, there remains a need for improved methods and machines for injection molding of plastic lures.